1. Field of the Invention
The invention relates to a shift control device and a shift control method of a vehicular continuously variable transmission that has a primary pulley, a secondary pulley and a belt wrapped around the two pulleys. More particularly, the invention relates to a shift control device and a shift control method of a vehicular continuously variable transmission equipped with a hydraulic circuit that has a pressure reducing valve for causing the ratio between the oil pressure in a primary hydraulic cylinder and the oil pressure in a secondary hydraulic cylinder to be in a predetermined relationship by causing a predetermined oil pressure to act on the primary hydraulic cylinder when the hydraulic oil supplying/discharging operation by a shift control valve is not performed.
2. Description of the Related Art
Among shift control devices of belt type continuously variable transmissions that have a primary pulley, a secondary pulley and a belt wrapped around the two pulleys, a well-known shift control device is equipped with a hydraulic circuit having a shift control valve that adjusts the amount of hydraulic oil supplied to or discharged from a primary hydraulic cylinder for altering the groove width of a primary pulley in order to shift the continuously variable transmission, and a pressure reducing valve that causes the ratio between the oil pressure in the primary hydraulic cylinder and the oil pressure in a secondary hydraulic cylinder for altering the groove width of a secondary pulley to be in a predetermined relationship by causing a predetermined oil pressure to act on the primary hydraulic cylinder when the hydraulic oil supplying/discharging operation by the shift control valve is not performed.
An example of the aforementioned device is a hydraulic control device of a continuously variable transmission described in Japanese Patent Application Publication No. JP-A-2005-42799. This patent application proposes a hydraulic control device of a belt type continuously variable transmission equipped with a hydraulic circuit capable of causing the speed change ratio to become a maximum speed change ratio by executing a so-called closing-in control of causing the ratio between the oil pressure in the primary hydraulic cylinder and the oil pressure in the secondary hydraulic cylinder to be in a predetermined relationship while causing a predetermined oil pressure to act on the primary hydraulic cylinder when the hydraulic oil supplying/discharging operation by the shift control valve is not performed. This hydraulic control device is designed to establish the maximum speed change ratio by the closing-in control at the time of launch from a stopped state of the vehicle so as to prevent the speed change ratio from changing toward the speed increase side (upshift side) and therefore realize good launching performance.
In the case where in a hydraulic circuit as proposed in Japanese Patent Application Publication No. JP-A-2005-42799, a shift is performed by a feedback control so that a target value and the actual value of the rotational speed of a predetermined rotating member or the like become equal to each other, the hydraulic oil supplying/discharging operation by the shift control valve ceases to be performed as the actual value approaches target value. Then, a predetermined oil pressure is caused to act on the primary hydraulic cylinder by the pressure reducing valve.
This operation, however, has a probability of causing a shift to be performed when there is no need for a shift. For example, at the time of a downshift by the feedback control, when the actual value approaches the target value, there is a possibility of the action of a predetermined oil passageway causing a sharp downshift toward the maximum speed change ratio. Furthermore, at the time of an upshift by the feedback control, when the actual value approaches the target value, there is a possibility that the supply of hydraulic oil to the primary hydraulic cylinder by the pressure reducing valve may cause a sharp upshift. Therefore, there is a possibility of giving rise to a problem of impairing the continuity of the control at the time of a shift.